JPH0555465B2 - - Google Patents

Description

[Detailed description of the invention]

FIELD OF THE INVENTION This invention relates generally to optical materials, and more particularly to methods of forming optically transparent yttrium oxide bodies. BACKGROUND OF THE INVENTION As is well known, there is a need for highly durable materials that are substantially transparent in both the visible and infrared regions. Applications of such materials include metal vapor lamps, optical windows, and aircraft optical imaging.
system). Optical imagers, such as those found in infrared heat-seeking missiles, are optical imaging devices such as windows or domes mounted on the exterior of the missile.
element). These external elements are provided to isolate the other optics of the imager from the external environment in which the missile flies. Therefore, these external elements must be particularly resistant to exposure to the external environment, have sufficient strength to protect other components during operation of the imaging device, and have visible As mentioned above, it is necessary to have substantial transparency in the light and/or infrared region. Several types of materials have been cited as promising candidates for such applications. Such materials typically each have high strength and may also theoretically exhibit a relatively high degree of infrared transmission, particularly in the wavelength range of about 2 microns to 5 microns. In addition to the aforementioned optical requirements for transparency in the wavelength range of approximately 2 to 5 microns, it is desirable for certain applications that the optical element be capable of transmitting wavelengths greater than 5 microns. . For example, the dome attached to the tip of a missile built to fly at extremely high speeds for long periods of time becomes hot. If the dome is made of a material that cannot transmit long wavelengths, e.g., greater than 5 microns, harmful emissions may occur, which may cause problems in the system of the optical imager shielded by the dome.
This will increase the noise. One requirement associated with such "hot" domes is that the dome material be able to withstand the thermal stresses resulting from aerodynamic surface heating. Some techniques for making Y ₂ O ₃ domes include sintering Y ₂ O ₃ bodies to substantially full density. In such techniques, up to 10% by weight of sintering aids such as La are added to achieve high densities. The addition of such sintering aids has one drawback. Adding the sintering aid to a material such as Y ₂ O ₃ reduces its thermal conductivity, which in turn reduces the thermal shock resistance of the dome, ultimately making the high temperature dome more susceptible to damage. (Means for Solving the Problems) According to the present invention, a method for forming a transparent yttrium oxide body includes preparing a powder made of yttrium oxide, and consolidating the powder into a predetermined size and shape. ) consists of the following steps. The compact is densified at high temperatures. The temperature ranges from 1700°C to at least 1700°C for a time sufficient to densify the compact to at least a closed porosity state.
It is best to keep it in the range of 1900℃, 1800 to 1900℃
It is even better if it is in the range of . Exemplarily, such closed pores in the yttrium oxide body are at least 91%, preferably from 94% to 96%, of the theoretical density of the yttrium oxide. The sintered, pore-closed object is heated at a high temperature, preferably in the range of 1700 to 1900°C, typically for 1/2 to 10 hours, for a desired temperature of 25,000 to 100°C.
By applying high pressure in the range of 30,000 psi (17.577 to 21.092 Kgf/mm ² ), it is possible to substantially
densified to %. This substantially fully densified body is annealed in air to regain oxygen lost from the body during exposure to the high temperature and pressure environment.
This special procedure ensures that the material is essentially 100% yttrium oxide, has a translucency of more than 70% over a wavelength range of about 2 to 6 microns, and has a wavelength of at least 7 microns. , a solid body with a translucency of at least 60% is obtained. The yttrium oxide body consists of substantially 100% yttrium oxide (at least 99.9% yttrium oxide). Therefore, this yttrium oxide exhibits high thermal conductivity and high thermal shock resistance by substantially transmitting light exceeding about 5 microns. Therefore, if the yttrium oxide body is used as a dome for a high-speed missile, the system will
Noise is sufficiently reduced. Looking at the present invention from another aspect, the method for forming a dome made of permeable yttrium oxide has an average particle size of 1 to 2.0 μm, a maximum agglomerated particle size of 10 μm,
The method includes providing a yttrium oxide powder having a yttrium oxide content of substantially 99.99%. The yttrium oxide powder is used in a press mold consisting of an aluminum mandrel coated with Teflon and a membrane of latex rubber or urethane rubber, which defines the shape of the dome. It is consolidated into a dome of predetermined size and shape by cold isostatic pressing using a mold. Powder is placed in the above mold, the mold is placed in an isostatic pressure press, and the pressure is set at 25000~30000psi (17.577~
Consolidate at a pressure in the range of 21.092 Kgf/mm ² ). The consolidated dome is heated at a temperature ranging from 1350 to 1450 °C for a specified period of time until it reaches approximately 75% of its theoretical maximum density.
Fired. This procedure is primarily used to remove the binder vehicle and dispersant. However, while exposed to a high temperature environment, the compressed dome becomes slightly more dense. The compressed and densified dome was then further densified or pore-closed to approximately 95% of its theoretical density by sintering in a vacuum furnace at temperatures ranging from 1700 to 1900 °C. be put into a state. This sintered, densified dome was heated for 25,000 hours, sufficient to reach a density substantially equal to 100% of the theoretical density of yttrium oxide.
It is subjected to high pressure argon gas of ~30000 psi (17.577-21.092 Kgf/mm ² ) and high temperature in the range of 1700-1900°C. During this final densification step, oxygen is lost from the workpiece (dome). This fully densified dome is heated in air to restore oxygen to it, rendering it transparent and thus stoichiometric. Exemplary, this heating is in the temperature range 1400-1800°C, preferably 1450°C
The holding time at the maximum temperature is 30 to 60 minutes depending on the size and thickness of the sample. (Function) Through the above special method, transparent yttrium oxide has high thermal shock resistance and high light transmittance (for a sample with a thickness of 2.8 mm, it exceeds 70% at wavelengths of 3.0 to 5.0 microns, and at wavelengths of 5 to 7 microns). 60%), and a relatively low absorption coefficient of less than 0.1 cm ^-1 can be obtained in the optical wavelength band of 3 to 5 microns. (Example) The production of a translucent object made of yttrium oxide will be described. consisting essentially of 99.9% pure yttrium oxide, preferably _99.99 % _Y2O3 ;
It has an average particle size of 1.0~2.0μm, and the maximum agglomerated particle size
Prepare 10.0 μm powder. Yttrium oxide powder having the above specifications is prepared as follows. Yttrium oxide powder is available from Union Moly Co., White Plains, New York, in a surface area range of 20-45 m ² /g and an average aggregate particle size of 1-3 microns. A typical example of this powder is
It is 99.99% pure yttrium oxide. This powder is placed in a polyethylene container or a rubberized container using a ceramic grinding medium such as zirconium oxide or yttrium oxide.
For example, ball mill for 5 to 24 hours using de-agglomerated media.
agglomerate). After milling, the slurry is passed through a 400 mesh sieve to remove large agglomerated particles to obtain a powder with an average particle size of 1-2.0 microns and a maximum agglomerated particle size of 10 microns. The powder can be further deagglomerated by spray drying with an ultrasonic horn. 3% by weight before spray drying
polyvinyl pyrrolidone (poly-vinyl pyrrolidone)
pyrolidone) (in a typical example, a ratio of 4:1 PVP40/
Add an organic binder vehicle such as PVP10 to loosen agglomerations. A small amount of a dispersing agent such as acetic acid is also added to the powder. The deagglomerated powder, with the addition of organic binder and dispersant, is applied to a spray dryer capable of drying the slurry at a rate of 1/hr. This spray drying method is commonly used to produce non-planar objects such as hemispherical domes. In non-planar molds such as those used for hemispherical domes, it is necessary to
It is advantageous to use self-flowing powders. Once the powder with the above specific particle size is obtained,
The powder is placed in a press mold suitable for producing a rough body of substantially the desired shape. For example, a press for a hemispherical dome.
The mold may include a Teflon-coated aluminum inner mold to form the concave surface of the dome and a urethane rubber membrane to define the convex surface of the dome. Powder is poured into the mold through the hole at the top, the hole is sealed, and air is removed from the mold. The entire mold is placed in an isostatic press and compressed at high temperature. Typically around 25000-30000psi
High pressures in the range (17.577-21.092 Kgf/mm ² ) are used. The rough dome is removed from the mold and heated in air at a temperature of 1350-1450°C to burn out the binder vehicle previously placed in the mold. The body is kept at that high temperature for up to 90 minutes in time. In a typical example, the density of the fired object is approximately 75% of the theoretical density, and the dimensions are
It has shrunk by about 10-12%. The final densification of the resulting coarse dome is carried out in two steps. In a first step, the coarse dome is densified to a density of about 95% of its theoretical density, or densified to obtain a body with a density greater than about 91% and with substantially closed pores. be converted into This is done by placing the object in a high vacuum tungsten furnace set at a temperature range of 1700-1900°C. Tungsten furnaces are used to reduce contamination of Y ₂ O ₃ bodies. In particular, in the case of a graphite type furnace, there is a possibility that the Y ₂ O ₃ body is significantly deprived of oxygen or carburized. This type of furnace (tungsten furnace) uses a reducing agent such as carbon to reduce the reduction of Y ₂ O ₃ by a chemical reaction, as in the graphite type furnaces traditionally used in the sintering process. used for The object is typically maintained at an elevated temperature for a period of up to 60 minutes. Preferably, the object is enclosed or shielded in a container made of Y ₂ O ₃ .
During this sintering process, a slight coating of tungsten may be deposited on the surface of the object, especially if the object is unprotected. This coating is usually physically removed before final densification. This sintered body has a theoretical density of 100%
% in the range of 1700~1900℃ and 25000~30000psi (17.577~21.092Kgf/ ^mm2 )
argon gas pressure for 5 to 10 hours, or until a density of substantially 100% of the theoretical density is reached. If anything, 1
It is desirable to heat at the highest temperature for a short time, but for a minimum time, so long as the treatment time is about 5 to 10 hours. When the object is placed in the high temperature graphite furnace used for this final densification, it is protected by a container made of Y ₂ O ₃ . This densification process makes the dome sufficiently dense (99.9% of theoretical density) and transparent. During the final densification stage, the dark bodies produced in the process due to the reduction of the material by an oxygen-free atmosphere are exposed to air in the furnace and regain oxygen. transparent
transparent). The object is heated from 1450℃ to approx.
It is exposed to temperatures of up to 1800°C, with holding times ranging from 30 to 60 minutes, depending on the size and thickness of the sample. This annealing renders the dome substantially transparent and it is then conventionally ground and polished to the desired surface finish and tolerances. For example, it can be polished by a fixed diamond polishing tool,
A specified surface finish can be achieved using alumina powder and high-speed lapping agents. Alternatively, the object is placed in an atmosphere of water vapor and hydrogen at a temperature ranging from 1400°C to a maximum of about 1800°C.
It may be held for 30 minutes and heat treated. However, a drawback of this method is that H ₂ may be incorporated, thereby creating harmful optical absorption bands. 1A and 1B, the relationship between wavelength and uncorrected in-line transmittance with respect to surface reflection loss is graphically shown for three types of samples having different thicknesses. The process frequencies for the three samples shown in Figures 1A and 1B and other samples of materials produced in accordance with the present invention are shown in the table.
parameter). In Figure 1A, curve 1
2 is thickness 0.040 inch (1.02mm) curve 14 is thickness
0.250 inch (6.35mm) and curve 16 is the thickness
The curve is for a 0.375 inch (9.53 mm) sample. These samples have a light transmission of at least 73% over the wavelength range of 2.5 to 6 microns and at least 60% above 6.5 microns (Figure 1A). Uncorrected in-line transmission percentages for surface reflection losses are shown for near UV, visible and infrared light. Curve 12' (Fig. 1B) corresponding to curve 12 of Fig. 1A is 1.1
At least 80% at ~2.6 microns, 0.6 to 1.1
Shows a transmission of at least 70% in the micron. Even the thickest samples show significant infrared transmission from 0.6 to 6.0 microns.

【table】

The yttrium oxide dome produced in accordance with the present invention is substantially pure, ie, essentially 99.9% yttrium oxide, and thus exhibits high thermal conductivity and associated properties. It also exhibits increased thermal shock resistance compared to dopant-added materials. FIG. 2 shows the relationship between temperature and thermal conductivity measured for a sample of Y ₂ O ₃ produced according to the present invention. (Effects of the Invention) In the present invention, sintering is performed only to obtain a closed pore state (between about 91% and 96% of the theoretical density). Therefore, since the process is performed at a lower temperature and in a shorter time than required by the conventional method used to obtain a completely dense sintered body, trace additives are required when making a completely dense sintered body. does not require. The use of a W (tungsten) heating element in the sintering process is considered advantageous in order to sinter the material to a closed pore state without using any trace additives. .
Additionally, the use of a tungsten furnace is beneficial in preventing the negative effects of carbon reduction on Y ₂ O ₃ that occur when graphite heating elements are used. Furthermore, in the yttrium oxide bodies produced according to this technology, the densification is carried out at a fairly low temperature, which is thought to accelerate material deterioration and particle growth. This is done by shortening the time of exposure to the environment. The average particle size of materials made in accordance with the present invention is generally about 150 microns. The absorption coefficient calculated for materials made according to the invention is approximately less than 0.1 cm ^-1 . This value was calculated for a wavelength of 4 microns using the data for curves 12 and 16 in Figure 1A and the formula below. β=ln(T ₂ /T ₁ )/t ₂ −t ₁ where T ₂ and T ₁ are the transmittance expressed in percent, and t ₂ and t ₁ are the thickness (cm). Although preferred embodiments of the invention have been described, it will be obvious to those skilled in the art that other embodiments incorporating these concepts may be practiced. It is therefore to be understood that these embodiments should not be limited to what is disclosed, but rather should be limited only by the spirit and scope of the appended claims. Embodiments of the invention are as follows. 1. An object containing at least 99.9% yttrium oxide having a density of at least 99% of the theoretical density, over a wavelength range of 2 to 5 microns when a sample of the object is 0.375 inches (9.53 mm) thick. %, the object has an average particle size of less than about 150 microns, and the object has an absorption coefficient of less than about 0.1 cm ^-1 at a wavelength of 4 microns. 2. An object containing about 99.9% pure yttrium oxide having an average particle size of about 150 microns and a density of at least 99% of the theoretical density, when a sample of the object is 0.040 inches (1.02 mm) thick.
at least 80 over a wavelength range of 5 microns
% in-line transmission, and the wavelength of the object is 4
The above object characterized in that the absorption coefficient in microns is less than about 0.1 cm ^-1 . 3. Said second having an in-line transmission of at least 76% over the wavelength range of 3 to 7 microns.
Objects mentioned in section. 4 Average particle size of about 150 microns, at least 15 W/M-〓 at 0℃, at least 15W/M-〓 from 0 to 300℃
Thermal conductivity of 7W/M−〓 and a purity of at least 99.9% of the theoretical density
An object consisting essentially of 99.9% polycrystalline yttrium oxide having an in-line transmission of at least 80% over a wavelength range of at least 2.5 to 6.8 microns when a sample of the object is 0.04 inches (1.02 mm) thick. The above-mentioned object is characterized by having a degree. 5 At least about 0.1 at a wavelength of 4.0 microns
Object according to paragraph 4 above, having an absorption coefficient of cm ^-1 . 6. A method for producing an object containing at least 99.9% yttrium oxide, the method comprising: preparing a raw material powder containing at least 99.9% yttrium oxide; compressing the yttrium oxide powder to produce an object containing at least 99.9% yttrium oxide; shaping; sintering the compacted object containing at least 99.9% yttrium oxide at elevated temperature to a closed porosity state;
preparing a body; at least 99.9% of the yttrium oxide pores are closed by heating the body at elevated temperature and pressure for a period sufficient to achieve a density of substantially 100% of the theoretical density; annealing an object having a density of substantially 100% of its theoretical density by heating the object in an atmosphere containing air to oxidize the object, thereby restoring a stoichiometric composition to the surface of the above method. 7. The method of item 6 above, wherein the raw powder has an average particle size of about 1.0 to 2.0 microns. 8. The method according to item 7 above, wherein the sintering step is performed in a high vacuum tungsten heating element furnace. 9 The densification process is performed at a temperature of 1700 to 1900°C,
The sixth step is carried out under increased pressure of 25,000 to 30,000 psi.
The method described in section. 10. The method according to item 9 above, wherein the temperature of the sintering step is 1700 to 1900°C. 11 The temperature of the sintering process is 1800 to 1900℃.
The method described in Section 10. 12. The method of paragraph 6 above, wherein the annealing step is performed at a temperature of about 1450°C. 13 A method for producing an object containing at least 99.9% yttrium oxide, having an average particle size of 1 to 2.0 microns and a maximum agglomerated particle size of 10 microns.
preparing a powder comprising at least 99.9% yttrium oxide, having a density of about 75% of the theoretical density;
A body containing 99.9% yttrium oxide is prepared; this 75% density body is heated under reduced pressure to
At a temperature of 1900℃, the holding time at maximum temperature is approximately 30
as a densified object with closed pores; the object is subjected to a temperature of 1700 to 1900°C under an argon gas pressure of 25,000 to 30,000 psi to substantially densify the object with closed pores. densifying the object to a density of 100% of the theoretical density; annealing the object in an air atmosphere at a temperature of 1400 to 1800°C for a holding time of 30 to 60 minutes at an elevated temperature; The above method. 14. The method of item 13 above, wherein the sintering step is performed in a high vacuum tungsten heating element furnace. 15. The method of paragraph 14 above, wherein the annealing step is performed at a temperature of about 1450°C. 16 A method for producing a non-planar object comprising at least 99.9% yttrium oxide having an in-line transmission of at least 80% over a wavelength range of 2.5 to 6.8 microns for a sample having a thickness of about 0.04 inches (1.02 mm), to prepare a yttrium oxide powder having an average particle size of 1 to 2.0 microns and a maximum agglomerated particle size of 10 microns; add a binder vehicle and a dispersant to the powder and spray dry the powder to form a free prepare a free-flowing powder; place the free-flowing powder in a non-planar mold;
Subjecting the powder to an isotropic static pressure of ~30,000 psi;
removing the binder vehicle at a temperature of ~1450<0>C;
The material is densified to a closed pore state using a holding time of less than 30 minutes at the maximum temperature; the resulting material is densified to a density of essentially 100% of the theoretical density under an argon pressure of 25,000 to 30,000 psi. densify the object with closed pores to a density of substantially 100% of the theoretical density by subjecting the object to a temperature of 1700 to 1900°C until The above method, characterized in that it includes the step of: annealing at a temperature of 1700 to 1900° C. for a holding time of 30 to 60 minutes at an elevated temperature. 17. The method according to item 16 above, wherein the non-planar body is a hemispherical dome. 18. The method of item 17 above, wherein the removing step is carried out until the object has a density of about 75% of the theoretical density. 19 Yttrium oxide is at least
15W/M-〓, at least 7W/M at 0-300℃
The object according to item 1 above, which has a thermal conductivity of -〓. 20 Yttrium oxide is at least
15W/M-〓, at least 7W/M at 0-300℃
The object according to item 2 above, which has a thermal conductivity of -〓.

[Brief explanation of the drawing]

Figures 1A and 1B show percent in-line optical transmittance in the visible (Figure 1B) and infrared (Figure 1A) samples of yttrium oxide prepared in accordance with the present invention.
Figure) is a graph shown as a function of wavelength over the wavelength range. FIG. 2 is a graph showing the relationship between temperature and thermal conductivity for Y ₂ O ₃ produced according to the techniques of the present invention.

Claims (1)

[Scope of Claims] 1. An object comprising at least 99.9% yttrium oxide having a density of at least 99% of the theoretical density, wherein a sample of the object has a thickness of 0.375 inches (9.53 mm).
has an in-line transmission of at least 73% over a wavelength range of 2 to 5 microns when the object has an average particle size of less than about 150 microns and the absorption coefficient at a wavelength of 4 microns is about 0.1 cm. The above-mentioned object characterized by being less than ^-1 . 2. An object containing about 99.99% pure yttrium oxide having an average particle size of about 150 microns and a density of at least 99% of the theoretical density, when a sample of the object is 0.040 inches (1.02 mm) thick. An object as described above having an in-line transmission of at least 80% over a wavelength range of 5 microns, wherein the object has an absorption coefficient of less than about 0.1 cm ^-1 at a wavelength of 4 microns. 3 Average particle size of about 150 microns, at least 15 W/M-〓 at 0℃, at least 15W/M-〓 from 0 to 300℃
A purity of at least 99.9 with a thermal conductivity of 7 W/M-〓 and a density of at least 99.9% of the theoretical density
% polycrystalline yttrium oxide having an in-line transmission of at least 80% over a wavelength range of at least 2.5 to 6.8 microns when a sample of the object is 0.04 inches (1.02 mm) thick. The above-mentioned object is characterized by having the following. 4. A method for producing an object containing at least 99.9% yttrium oxide, the method comprising: preparing a raw material powder containing at least 99.9% yttrium oxide;
forming an object containing 99.9% yttrium oxide; sintering the compacted object containing at least 99.9% yttrium oxide at elevated temperature to prepare an object in a closed porocity state; ; At elevated temperature and pressure, an object is reduced to substantially its theoretical density.
Densify an object with at least 99.9% pores of yttrium oxide closed by heating for a period sufficient to achieve 100% density; by heating the object in an atmosphere containing air. , annealing the object having a density substantially 100% of the theoretical density to oxidize the object, thereby restoring stoichiometric composition to the surface of the material; Method. 5. A method for producing an object containing at least 99.9% yttrium oxide, having an average particle size of 1 to 2.0 microns and a maximum agglomerated particle size of 10 microns.
preparing a powder comprising at least 99.9% yttrium oxide, having a density of about 75% of the theoretical density;
% of yttrium oxide; this 75% density object was heated under reduced pressure to
The object is densified and has closed pores at a temperature of 1700-1900°C under an argon gas pressure of 25,000-30,000 psi with a holding time at the maximum temperature of about 30 minutes. , the object with its pores closed has practically the theoretical density.
densifying the object to 100% density; annealing the object in an air atmosphere at a temperature of 1400 to 1800°C for a holding time of 30 to 60 minutes at an elevated temperature; Method. 6. A method for producing a non-planar object comprising at least 99.9% yttrium oxide having an in-line transmission of at least 80% over a wavelength range of 2.5 to 6.8 microns for a sample having a thickness of about 0.04 inches (1.02 mm), to prepare a yttrium oxide powder having an average particle size of 1 to 2.0 microns and a maximum agglomerated particle size of 10 microns; add a binder vehicle and a dispersant to the powder and spray dry the powder to form a free prepare a free-flowing powder; place the free-flowing powder in a non-planar mold;
Subjecting the powder to an isotropic static pressure of 30,000 psi;
removing the binder vehicle at a temperature of 1450°C; densifying the powder to a closed pore state under reduced pressure at a temperature of 1700-1900°C with a holding time at maximum temperature of less than 30 minutes; The pores of the object are closed by subjecting the object to a temperature of 1700 to 1900° C. under an argon pressure of 25,000 to 30,000 psi and to a density of substantially 100% of the theoretical density. densify the object to a density of substantially 100% of its theoretical density; annealing the object at a temperature of 1700 to 1900°C in an atmosphere containing hydrogen and water vapor with a holding time of 30 to 60 minutes at elevated temperature; The method described above, comprising the steps of;